Displacement sensing based on intensity detection by a SNAP-taper coupling system

A novel highly sensitive displacement sensing method based on the intensity detection of the resonant spectrum, which arises from a fiber-based surface nanoscale axial photonics (SNAP) resonator, is proposed. By means of dissipative and dispersive coupling mechanisms, the changes of the relative position between the SNAP resonator and fiber taper not only bring the shift in a resonant wavelength, but also lead to the variation of the linewidth and extinction ratio of the whispering gallery mode (WGM) in spectrum. Instead of the wavelength shift, we utilize the extinction ratio changes to realize the displacement sensing, which is robust against lasing and microresonator frequency noise in the detecting system. Using the analytical expression of the transmission spectrum, the extinction ratio as a function of the displacement for different axial modes is obtained. It is proved that a large range and high resolution displacement sensor can be achieved by simultaneously tracking the extinction ratio of multiple axial modes. The fiber-based SNAP resonator can be fabricated into a probe-type sensor, making it potential and a powerful tool for many displacement sensing applications such as microstructure measurements in both aerospace and nano-lithography fields.